Of various interfaces available for interacting with a computer system one of the easiest to use and understand is the touch screen. This technology allows a user to simply touch an icon or picture to navigate through the system, display the information the user is seeking, and to enter data. For this reason this technology is widely used in many areas, including bank machines, information kiosks, restaurants, cars, etc.
A number of different methodologies are used to implement touch screen technology, and each has advantages and disadvantages. The three main types of technology used are resistive, capacitive and surface acoustic wave.
Resistive technology uses a flexible membrane that is affixed over a display. The membrane and display each have a conductive layer, and typically the membrane is energized with an electrical potential. When the membrane is touched, it is brought into contact with the conductive layer on the display, and this creates current flow. Various sensors around the display measure the current and a controller can determine, either through an absolute value or through a ratio with the current measured at other sensors, the location of the touch. One example of this technology is found in U.S. Pat. No. 4,220,815 to Gibson et al.
One of the advantages of resistive touch screens is that they can be pressed by either a finger or a stylus. The technology responds to pressure and the pressure can be exerted by anything. This is important in some cases where a user may wish to press the screen with the back of a pen or other stylus, with fingernails or with gloved hands.
A second advantage is that they are sealed and not affected by dirt. Thus they can for example be used in industrial applications where the user's hands may be greasy or dirty. Further, the touchscreen will work irrespective of whether there is dust or grime on the screen or in the area around the periphery of the screen.
This technology will also continue to work even when scratches exist on the outer surface of the membrane.
The main disadvantage of resistive touch screens to date has been the material from which the flexible membrane has been made. The requirement that the membrane be flexible and resistant to breakage has generally meant that polyester films have been used. The problem with these films is that they are easily scratched, torn and melted, and are thus susceptible to vandalism or inadvertent damage. This has generally limited the use of this technology to applications where access to the screens is restricted, and where the general public is not given access to these machines. For example, information kiosks in shopping malls or airports do not typically use resistive touch screens due to the vandalism potential.
A second technology for touch screens is capacitive. In this technology a layer of glass is used as a dielectric, and typically has a sensor grid on its lower surface. The touch of a user creates a change in capacitance that can be measured by the sensor grid, allowing the controller to determine when and where a touch occurs.
The advantage of capacitive touch screens is that their outer layer is glass, and thus more resistant to vandalism and damage.
One disadvantage of capacitive touch screens is that they can be susceptible to electromagnetic interference, and can thus produce false hits. This interference can be caused by a number of things, but most commonly in public locations by cellular telephones and pagers. Due to this potential interference, capacitive touch screen cannot be used in certain applications such as in some military equipment.
A second disadvantage is that the sensitivity of the screen can be affected by dirt and scratches. These change the capacitance that is sensed, and can create false touch signals.
Another disadvantage is that skin must be used to make contact with the display. A stylus, fingernail or gloved hand will not produce a sensed touch. Further, in some cases dry hands may not create a sensed touch.
A third technology that is used is the surface acoustic wave. In this technology ultra-sonic waves are emitted onto the surface of the screen, and microphones situated around the screen detect these waves. The periphery of the screen is generally reflective to the waves. When the screen is touched the waves are affected, and a controller is able to determine the location of the touch based on the information received by the microphones.
The major problem with this technology is that it is susceptible to dust and dirt. Any particle will affect the waves. Further, when these types of screens are cleaned, the dirt may be pushed to the periphery, where it will affect the reflective surface. The result of the dirt is that a touch may be perceived to be in a different location than the actual touch location.
What is therefore needed is a touchscreen technology that is robust, so that it can sense the touch of a finger, gloved hand, or any stylus. Further, the technology is required to be unaffected by dirt and scratches. Also, the outer touch surface must be hard and resistant to vandalism.